<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Hello World</title>
<link rel="stylesheet" type="text/css" href="//netdna.bootstrapcdn.com/bootstrap/3.1.1/css/bootstrap.min.css">
</head>
<body>
<nav class="navbar navbar-default">
<div class="container-fluid">
<!-- Brand and toggle get grouped for better mobile display -->
<div class="navbar-header">
<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
<span class="sr-only">Toggle navigation</span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
<span class="icon-bar"></span>
</button>
<a class="navbar-brand" href="#">某菜单管理系统</a>
</div>
<!-- Collect the nav links, forms, and other content for toggling -->
<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
<ul class="nav navbar-nav">
<li class="active"><a href="#">首页 <span class="sr-only">(current)</span></a></li>
<li class="dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">功能 <span class="caret"></span></a>
<ul class="dropdown-menu">
<li class="dropdown-header">业务功能</li>
<li><a href="#">信息建立</a></li>
<li><a href="#">信息查询</a></li>
<li><a href="#">信息管理</a></li>
<li role="separator" class="divider"></li>
<li class="dropdown-header">系统功能</li>
<li><a href="#">Separated link</a></li>
</ul>
</li>
<li><a href="#">帮助</a></li>
</ul>
<form class="navbar-form navbar-right">
<div class="form-group">
<input type="text" class="form-control" placeholder="用户名">
<input type="password" class="form-control" placeholder="密码">
</div>
<button type="submit" class="btn btn-default">登录</button>
</form>
</div><!-- /.navbar-collapse -->
</div><!-- /.container-fluid -->
</nav>
<script src="http://cdn.bootcss.com/jquery/3.1.0/jquery.min.js"></script>
<script src="http://cdn.bootcss.com/bootstrap/3.2.0/js/bootstrap.min.js"></script>
<script src="https://cdn.bootcss.com/popper.js/1.12.5/umd/popper.min.js"></script>
</body>
</html>
head头部的样式引用。href内容添加“http:”就出来了，网页F12打开控制台，会报错：原因就是找不到样式